The invention relates to the removal of water, carbon dioxide and nitrous oxide, and optionally also hydrocarbons, from an air stream prior to cryogenic air separation.
The cryogenic separation of air requires a pre-purification step for the removal of both high-boiling and hazardous materials. Principal high-boiling air components include water and carbon dioxide. If removal of these impurities from ambient feed air is not achieved, then water and carbon dioxide will freeze out in cold sections of the separation process, such as heat exchangers and the LOX sump. This will cause pressure drop, flow variations and operational problems. Various hazardous materials have also to be recovered including acetylene and other hydrocarbons. The high boiling hydrocarbons, if not removed, will concentrate in the LOX section of the column, resulting in a potential explosive hazard.
It is known that oxides of nitrogen should be removed also. A minor air component is nitrous oxide N2O, which is present in ambient air at about 0.3 ppm. It has similar physical properties to carbon dioxide and therefore presents a potential operation problem because of solids formation in the column and heat exchangers of the cryogenic distillation apparatus. In addition, nitrous oxide is known to enhance combustion of organic materials and is shock sensitive. As such, nitrous oxide also presents a safety hazard. Hydrocarbons such as ethylene, acetylene, butane, propylene and propane are further impurities which are desirably removed prior to cryogenic air separation.
The pre-purification of air is usually conducted by adsorptive clean up processes. These may operate by thermal swing adsorption (TSA) as described in U.S. Pat. Nos. 4,541,851 and 5,137,548 or U. Gemmingen (“Designs of Adsorptive Driers in Air Separation Plants” Reports on Technology 54/1994, Linde), by pressure swing adsorption (PSA) as described in U.S. Pat. Nos. 4,711,645, 5,232,474 or C. W. Skarstrom (“Heatless Fractionation of Gases over Solid Adsorbents” vol II, 95, N. W. Li (Ed), CRC Press, Cleveland, Ohio 1972), or by variants of those processes such as thermally enhanced PSA (TEPSA) as described in U.S. Pat. No. 5,614,000 or TPSA as described in U.S. Pat. No. 5,855,650.
In general, pre-purification of air is carried out by adsorption of contaminating gas components from the air by adsorption on a solid adsorbent with periodic regeneration of the adsorbent. In such methods, the air is fed in contact with at least two layers of solid adsorbents to adsorb the components to be removed, the concentration of which components gradually increases in the adsorbents. The concentration of each of the removed gas components in the adsorbent will not be uniform but will be highest at the upstream end of the adsorbent bed and will tail off progressively through a mass transfer zone in the adsorbent. If the process is conducted indefinitely, the mass transfer zone will progressively move downstream in the adsorbent bed until the component which is to be removed breaks through from the downstream end of the bed. Before this occurs, it is necessary to regenerate the adsorbent.
In pressure swing adsorption (PSA) systems, this is done by stopping the flow into the adsorbent of the gas to be treated, depressurising the adsorbent and, usually, by passing a flow of a regenerating gas low in its content of the component adsorbed on the bed through the bed counter-current to the product feed direction. As the component which is being removed is adsorbed while the bed is on-line, the adsorption process will generate heat of adsorption causing a heat pulse to progress downstream through the adsorbent. During the regeneration process, heat must be supplied to desorb the gas component which has been adsorbed on the bed. In PSA, one aims to commence regeneration before the heat pulse has reached the downstream end of the bed; the direction of travel of the heat pulse is reversed by the counter-current flow of the regenerating gas and the heat derived from the adsorption of the gas component in question is used for desorbing that component during regeneration. One thus avoids having to supply heat during the regeneration step. However, the short cycle time (feed time of typically 10-15 min) used in order to avoid the heat pulse leaving the adsorbent bed requires frequent depressurisation of the bed, during which the feed gas is vented off and lost (“switch loss”). In addition, it is usual to use two adsorbent beds, with one being on-line while the other is regenerated. The depressurisation and regeneration of one bed must take place during the short time for which the other bed is on-line, and rapid repressurisation can lead to transient variations in the feed and product flows which can adversely affect plant operation.
An alternative procedure is known as temperature swing adsorption (TSA). In TSA, the cycle time is extended (feed time of typically 2-12 h) and the heat pulse mentioned above is allowed to proceed out of the downstream end of the adsorbent bed during the feed or on-line period. To achieve regeneration, it is therefore necessary to supply heat to desorb the adsorbed gas component. To this end, the regenerating gas used is heated for a period to produce a heat pulse moving through the bed counter-current to the normal feed direction. This flow of heated regenerating gas is usually followed by a flow of cool regenerating gas which continues the displacement of the heat pulse through the bed towards the upstream end. TSA is characterised by an extended cycle time as compared to PSA. TSA is energy intensive because it is necessary to supply regenerating gas heated to a high temperature such as 150-200° C. in order to ensure desorption of the more strongly adsorbed component from the bed. It is usual also to pre-cool the air to be treated in order to minimise the amount of water that must be adsorbed on the bed, further increasing plant and energy costs.
In a typical air pre-purification TSA method, a two-layer bed is employed to remove essentially all of the water and carbon dioxide present in the feed air stream. Since water is the more strongly adsorbed of the two species, the beds are usually run until carbon dioxide starts to break through the adsorbent bed. More CO2 than N2O is present in the feed air stream, but since 13X has a larger capacity for CO2 than for N2O, if the beds are run to CO2 breakthrough, significant amounts of N2O will break through from the bed, and may cause problems downstream in the cryogenic distillation plant.
U.S. Pat. Nos. 4,249,915 and 4,472,178 disclose an adsorption process in which moisture and carbon dioxide are removed from atmospheric air by adsorption in separate respective beds. The moisture laden bed is regenerated by PSA in a relatively short operating cycle, while the carbon dioxide laden bed is regenerated thermally at considerably longer time intervals. While there are certain benefits to this arrangement, the plant costs are high due to duplication of columns and the need for additional equipment to carry out both systems of regeneration of the respective beds.
Wenning (“Nitrous oxides in Air Separation Plants” U. Wenning, Proceedings from MUST 96, pp 79-89) describes how carbon dioxide can displace already adsorbed nitrous oxide from a zeolite adsorbent, causing breakthrough of nitrous oxide at a concentration greater than that in ambient air.
U.S. Pat. No. 5,919,286 teaches that a layer of zeolite at the product (downstream) end of an alumina bed can be used for nitrogen oxides removal in a PSA process.
EP0992274 describes a process for the removal of carbon dioxide, water and nitrous oxide from air preferably in a TSA process, in which a three-layer adsorbent bed is used, with a first layer, for example of alumina, primarily adsorbing water, a second layer, for example of 13X, primarily adsorbing carbon dioxide, and a third layer, for example of CaX, primarily adsorbing nitrous oxide.
U.S. Pat. No. 5,846,295 describes a TSA process for the removal of CO2 and H2O in which impregnated alumina is used, in some cases in combination with a zeolite such as 13X at the product end of the bed.
U.S. Pat. No. 5,614,000 describes a process for removal of water and CO2 from air in which an adsorbent bed, preferably containing only alumina, may be regenerated partially by TSA and partially by PSA, with the part of the adsorbent that adsorbs water (the upstream part) being regenerated by PSA whereas the remainder is regenerated by TSA using a regenerating gas temperature of around 70° C. Such a process is known by the acronym TEPSA.
U.S. Pat. No. 5,855,650 describes a process for removal of water and CO2 from air in which an adsorbent bed containing a layer of alumina and a layer of 13X zeolite, or a single layer bed entirely of alumina, is regenerated by TSA using a gas temperature of around 100° C. in the downstream part, whereas the upstream part on which water is adsorbed is regenerated partly by TSA and partly by PSA. Such a process is known by the acronym TPSA.
The selectivity for one gas compared with that for another gas exhibited by an adsorbent can be expressed as the ratio of the Henry's Law constants (initial isotherm slopes) for the two gases at 30° C.